This invention concerns a method of stabilizing cation-exchange resins against oxidative degradation. 871-899
Cation-exchange resins are widely employed by industry to remove impurities from liquid mixtures, or to separate various components contained in such mixtures. Conventional ion-exchange resins are prepared by functionalizing a copolymer matrix with groups having anions or cations which are capable of being exchanged for, or associated with, ions or molecules having the same charge when the resin is in contact with a liquid mixture. Such resins have generally been used in water treatment and purification, food preparation, pharmaceutical manufacturing, chemical processing, metal extraction, and so on, as is generally discussed by R. M. Wheaton et al. in, "Ion Exchange", 11 Kirk-Othmer Ency. Chem. Tech. pp. 871-899 (2nd Ed. 1966).
One problem associated with conventional cation-exchange resins is oxidative degradation of the copolymer matrix over an extended period of time. Liquid mixtures which are typically in contact with the resin inherently contain quantities of oxidizing species such as molecular oxygen, or may have an elevated temperature, both of which may promote more rapid degradation of the copolymer matrix. It is believed that during oxidative degradation, carbon-carbon bonds rupture with respect to cross-links between individual polymer chains and/or links between individual styrene moieties. The loss of such bonds results in an increase in water retention capacity and, ultimately, in the release of organic contaminants, such as segments of functionalized linear polystyrene. As used hereinafter, the term "oxidative degradation" refers to such degradation of the copolymer matrix.
Degradation of the copolymer matrix is undesirable for commercial operation of an ion-exchange or chromatographic process. For example, resins which decross-link become relatively soft and swell to a greater extent. Such changes ultimately result in an increased bed pressure drop, a reduced flow rate for the liquid mixture being treated, and reduced operating capacity for removing chemical species when such resins are employed in a typical ion-exchange or chromatographic column. Additionally, the release of organic contaminants into column effluent can be unacceptable in some applications, such as those used in preparing food products. Organic contaminants may also create a source of potential corrosion to process equipment. Organic contaminants may also foul anion-exchange resins associated with a process, such as in a mixed-bed application where both cation- and anion-exchange resins are mixed together, or when an anion-exchange resin is employed in a subsequent process step.
Previously, industry attempted to remedy oxidative degradation and its associated problems by increasing the amount of cross-linking monomer used in preparing the copolymer matrix. However, an increase in the number of cross-links renders the resulting resin bead less compatible with liquid mixtures, thereby resulting in reduced diffusion into the bead and poor operating capacity. A highly cross-linked resin also generally exhibits poor regeneration efficiencies and may be impermeable with respect to large molecules, such as glucose, fructose, and other sugars. Furthermore, increasing the cross-link density does not address problems associated with release of organic contaminants, since degradation still occurs.
U.S. Pat. No. 3,342,755 issued to Calmon et al. discloses a postulated mechanism for degradation of the copolymer matrix by identifying a so-called "weak link" at tertiary carbons adjacent to the benzene ring of a styrene moiety. This weak link is said to exist, due to the tendency of hydrogen attached to the tertiary carbons to form hydroperoxides with oxidizing agents like molecular oxygen or chlorine. The hydroperoxides are said to eventually lead to splitting of carbon chains associated with the copolymer. Calmon et al. attempt to solve the problem by substituting a halogen for the hydrogen at the tertiary carbon, and further disclose that nuclear halogenation, i.e., employing a monomer such as ortho-chlorostyrene, does not contribute to resin stability.
As can be seen, it is desirable to develop cation-exchange resins which have improved resistance to oxidative degradation. Use of such resins in an ion-exchange or chromatographic separation process would promote a more efficient and reliable process.